170 THE MICROSCOPE. 



as without it the transmitted rays would in many cases 

 produce confusion of the image. Dr. Anthony advo- 

 cates the use of a stage- diaphragm, and which consists, 

 as seen in fig. 100, of three slips of smooth blackened 

 cardboard or vellum with perforations, any of which 

 can be brought into the centre and clamped, and re- 

 tained in its place under the glass slip. The larger 

 perforated discs form an additional slide ; while various 

 other forms, slits, slots, cat's-eyes, bars, &c., may be 

 added at pleasure. 



The Iris Diaphragm, fig. 101, is an inexpensive and 

 ingenious form of iris diaphragm designed by Wale, of 

 America, for use with his "Working, or Student's," 

 microscope. It consists of a piece of very thin cylindrical 

 tube, A, about f of an inch in length and | of an inch in 



FIG. 101. Wale's Iris Diaphragm. 



diameter, the circumference of which is cut throughout 

 with shears to nearly the whole length, and at intervals 

 of about of an inch; by means of a screw collar 

 B attached below, this cut tube is forced into a 

 parabolic metal shsll, contained within c, whose 

 apex is truncated to an aperture of about -| of an inch; 

 the pressure of the screw causes the thin metal tongues 

 to turn and to overlap in a spiral, which gradually dimin- 

 ishes the aperture to the size of a pin-hole. On unscrew- 

 ing the collar B, the spiral overlapping of the tongues 

 is released, and by their elasticity causing the aperture 

 gradually to expand. The whole device is fitted into the 

 opening of the stage from beneath, so as to be flush with 

 the upper surface, with one turn of a coarse screw on 

 the edge of c. 



